Dwarf seahorses are stealth hunters of the oceans

One of the most effective predators in the animal kingdom is smaller than your thumb and won’t win any races.

The dwarf seahorse is just 2.5 centimetres long, and thanks to its S-shaped body and small dorsal fin, it’s going nowhere fast. But oddly enough, it is this unusual body shape and lack of speed that makes the seahorse a menacing hunter.

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Unlike fish with protruding jaws, the seahorse has a long, thin snout that it rotates toward prey in a swift snatching motion called pivot feeding. This millisecond manoeuvre creates suction that pulls in the seahorse’s prey, but it only works at extremely close range.

“We knew that these seahorses were feeding successfully by doing this short-range swinging motion,” says marine biologist Brad Gemmell at the University of Texas in Port Aransas. “But they must first overwhelm the ability of the prey to escape. Our question was, how do they get so close without alerting their prey?”

This is no mean feat. Copepods, the tiny crustaceans that dwarf seahorses eat, are highly sensitive to changes in the water around them. They rely on small sensitive hairs to detect motion, and once they sense danger they have one of the fastest escape responses of any organism on the planet – they are able to flee at 500 body lengths per second. A cheetah can only manage 30 body lengths per second.

Why the long face?

Gemmell and a team of researchers used high-speed cameras to measure the movements of seahorses and the velocity of the water around them as they stealthily approached their prey. They found that the water just above the snout of the seahorse was significantly less turbulent than above or to the side its body. As a seahorse orients itself toward its prey, the calmer water directly above its mouth allows it to sneak up and pounce.

Once the creature is in ideal striking range, the seahorse’s mouth covers the distance to the copepod in less than a millisecond, giving it little chance of escape. In Gemmell’s observations, seahorses that were able to get within a millimetre of the copepods successfully caught them 79 per cent of the time. They were far more successful when they approached slowly.

Adam Jones at Texas A&M University in College Station studies seahorse and pipefish behaviour. He says this stealthy behaviour is a big part of a seahorse’s way of life. “Prey seem entirely oblivious to the seahorse’s presence until it’s too late, probably because the seahorses move so little while they forage”.